This invention relates to a color cathode ray tube (CCRT) having an arc suppression structure to minimize surge currents from the CCRT caused by internal arcing, and more particularly relates to such a CCRT having feedback features for an auto-convergence system.
An automatic convergence system has recently been developed for high resolution CCRT displays expected to have application in such demanding fields as computer aided design (CAD) and cartography. See ELECTRONIC PRODUCTS, May 12, 1983, p. 17. Essential to such an auto-convergence system are certain feedback features in the CCRT, which provide information on the location of the scanning electron beams to a computer, which then corrects any misconvergence of the beams. Such feedback features include a phosphor pattern on the back or gun side of the tube's aperture mask, and a window in the side of the tube. When struck by the scanning electron beams, the phosphor pattern emits radiation, some of which is transmitted through the window and detected by an externally placed photomultiplier tube.
The window must not only be transparent to the emitted radiation but also must be sufficiently conductive to prevent localized charge build-up, which could distort the adjacent potential field, resulting in disturbance of the electron beam paths. A suitable window structure is described and claimed in co-pending U.S. patent application Ser. No. 448,468, base issue fee paid Apr. 15, 1985, and assigned to the present assignee.
In addition the getter flash, a deposit of gas-adsorbing material essential to adequate life of the CCRT, must be distributed in a manner to avoid both the phosphor pattern on the back of the mask and the window, to assure an adequate signal to the photomultiplier tube. Such a distribution is achieved in a getter structure described and claimed in U.S. patent application Ser. No. 449,897, filed Dec. 15, 1982, also assigned to the present assignee.
However, the feedback CCRT shares a common problem with other CCRTs, that is, susceptibility to high surge currents caused by internal arcing. Such susceptibility to arcing is not surprising in view of typical operating potentials as large as 25 to 30 kilovolts, and the large potential differences between various tube components, especially the closely spaced gun electrodes. Steps are taken during manufacture to minimize arcing during later tube operation, especially the step of high voltage conditioning in which a voltage of 40 kilovolts or more is applied between the terminal high voltage electrode and the adjacent electrode of the electron gun to remove projections and foreign matter from the inter-electrode spacing. Despite this and other precautions, occasional arcing does occur, resulting in momentary surge currents as high as 400 amps, which can be devastating to electrical components outside the CCRT. Thus, numerous structures have been proposed to reduce or dissipate such surge currents inside the CCRT. (For internal high resistance coatings in the neck and funnel regions of the tube, see, for example, U.S. Pat. Nos. 2,829,292; 3,555,617; 3,961,221; 3,959,686; 4,249,107; 4,280,931 and German Pat. No. 2,634,102). (For discrete resistors between the getter wand and the gun, see, for example, U.S. Pat. Nos. 3,355,617; 4,101,803 and 4,255,689). (For a spark gap across such a resistor, see U.S. Pat. No. 4,234,816). (For discrete resistors between the internal conductive coating and the convergence cup, see, for example, U.S. Pat. No. 3,295,008 and British Pat. Nos. 1,353,872 and 1,448,223). (For discrete resistors between various gun components, see U.S. Pat. No. 4,345,185 and Japanese application No. 40,12432). (For getters placed to avoid shorting of the internal coating or discrete resistors, see, for example, U.S. Pat. Nos. 3,979,633; 4,182,974; and 4,230,966).
High resistance coatings in the neck region can be effective "surge limiters" by suppressing arc currents during tube operation, but such coatings hinder high voltage conditioning during processing. U.S. Pat. No. 3,959,686 addresses this problem by placing the high resistance coating between two lower resistance coatings in the neck and the mask-screen areas, respectively. Two anode buttons are provided in the tube wall, one conventionally placed in the upper (low resistance) coating to provide the tube operating potential, and the other placed in the lower (low resistive) coating to provide the conditioning potential. Thus, the middle (high resistance) coating can function as a surge limiter without interfering with conditioning. However, such a tube structure is complex and expensive to produce.
Another problem associated with high resistance coatings in the neck region is that their effectiveness may be reduced or eliminated by the getter assembly or getter flash or both forming a conductive bridge across the coating. Solutions offered to avoid this problem include moving the getter away from the neck region, for example, to the mask (U.S. Pat. No. 3,979,633), and placing a resistor between the getter wand and the gun convergence cup (U.S. Pat. No. 4,101,803). Moving the getter to the mask results in getter flash deposits on the back side of the mask. This is, of course, undesirable in the feedback CCRT. Placing a resistor between the wand and the cup risks shorting of the resistor, unless the getter flash is directed away from the resistor, i.e., toward the mask (U.S. Pat. No. 3,355,617).